During most cases of hip arthroplasty, both bearing elements of the ball and socket joint are replaced with manufactured implants in the form of an acetabular component and a femoral component. There are various designs of these components and they may be manufactured in various materials, have various combinations of geometry and be implanted and fixed by various means.
The acetabular component is usually in the form of a concave partial sphere which functions as a cup or socket. It is fixed into place in a prepared acetabulum by one of a number of means. The femoral component is usually in the form of a convex partial sphere (i.e. a ball), which is fixed in place on the top of a prepared femur, also by various means. The femoral component is shaped so as to be received within the acetabular component to form the joint.
When engaged, it is preferable that the position of the centre of rotation of the bearing elements approximates the centre of rotation of the original anatomical bearing otherwise there will be an increased risk of hip dislocation. During installation of such a bearing system, it is also desirable to place the elements at such positions so as to optimise their performance. This performance may be expressed in terms of a series of factors such as longevity, amount of wear, morphology, size of wear debris, range of motion, stability in motion, and frictional torque. The relative importance of these performance factors is implant and patient specific. However, all performance factors are influenced in various degrees by the intra operative positioning of each element of the bearing—relative to each other and relative to the patient's anatomy.
With regard to total knee replacement it is known that varus positioning of the tibial component can deteriorate performance characteristics. Increased wear of the bearing, increased fracture rate of the tibial base plate and increased loosening rate of the tibial component are all recognized problems with tibial component mal-position.
A company called BrainLAB provides a particular device for use in positioning such bearing components. This device employs computer simulation and navigation software and uses multiple infrared sources and cameras to detect the position of a pointer instrument operated by a surgeon. The computer is configured to display an image of the patient's anatomy and to direct the surgeon to position the implant in the correct place. Others have also proposed use of computer-aided navigation in a similar manner to that described but involving computerised tomography (CT) or the morphing standard models. There are several drawbacks associated with such systems, including the large cost of the equipment, the complexity of the system, the time involved in the positioning procedure and the fact that it requires the surgeon to continually glance at a computer screen and therefore take his eyes away from the patient during the operation. Primarily due to the expense of the above devices, most surgeons have no option but to align tools and implants by eye. This can often lead to later complications since the positioning of such elements is uncontrolled and generally inaccurate.
It is therefore an aim of the present invention to provide an alignment device, which addresses some or all of the aforementioned problems.